The present-day toolmaking practice has as one of its leading trends the attainment of higher durability of cutting tools by depositing wear-resistant coatings thereon, said coatings being made of interstitial phases based on metals of Groups III through VI of the Periodic System. Some of these metals, e.g., Zr, Hf, La, Al, Fe, Co, Mn, when cooled from the temperatures of the order of 1000.degree. C. down to room temperature, are liable to undergo polymorphic transformations, or to form interstitial phases of the various modifications which are also capable of polymorphic transformations when exposed to cooling. It is common knowledge that all polymorphic transformations of metals and interstitial phases are accompanied by a change in their lattice structure, which results in stresses arising therein, in alteration of their physical properties, e.g., specific volume, magnetic hysteresis, and in impaired mechanical properties, in particular, plasticity.
When the aforesaid metals or interstitial phases undergo crystallization from a liquid or vaporous state, some residual unstable modifications are present in their structure that have not undergone polymorphic transformations. Non-homogeneous phase composition leads, in turn, to unstable mechanical characteristics of such metal and interstitial phases.
Known in the art is a method for production of a cemented-carbide item provided with a wear-resistant coating based on an interstitial phase (aluminium oxide) liable to undergo polymorphic transformations when cooled (cf. U.S. Pat. No. 3,967,035, Int. Cl. C23C 11/08, published June 29, 1976). Said wear-resistant coating is deposited upon the cemented-carbide article by the gaseous-phase method which is characterized by high heating temperatures (900.degree. to 1250.degree. C.). The article being processed is kept at the above temperatures for one to three hours in an atmosphere of aluminium gallide, steam and hydrogen. To establish a wear-resistant coating from the alpha-modification alumina, the steam-to-hydrogen ratio should be within 0.025 to 2.0. The aforesaid alpha-modification exhibits maximum stability to heating up to 2000.degree. C., and is free from admixtures of alkali metals which are present in other less stable modifications of alumina.
The method discussed above is instrumental in depositing a wear-resistant coating made from interstitial phases of transition metals, upon a cemented-carbide article. However, the method is inapplicable for depositing such a coating upon a cutting tool made of steel which features a lower unhardening point as compared with cemented carbides.
Another prior-art method for making cutting tools from an iron-based alloy provided with a wearresistnt coating based on interstitial phases is known (cf. the journal `Physics and chemistry of materials treatment`, Nauka Publishers, No. 2, 1979, pp. 169 to 170). In this method a bias voltage is applied to the cutting tool placed in a vaccum chamber, the tool is heated and cleaned up by being bombarded with the ions of the cathode material evaporable by an arc discharge, whereupon the bias voltage is reduced to a value at which a wearresistant coating is formed, and the predetermined-thickness wear-resistant coating is established due to interaction of the cathode material being evaporated with a reactant gas admitted to the vacuum chamber, after which the cutting tool is annealed.
According to the method discussed above, a molybdenum-based alloy is, used as the cathode material which is not liable to develop polymorphic transformation upon being cooled, and the wear-resistant coating is formed from molybdenum carbide. However, molybdenum carbide features but low oxidation resistance, heat resistance and thermal conductivity, inadequate thermodynamic stability, as well as is liable to decompose at temperatures below its melting point. As a result, the molybdenum-carbide coating exhibits insufficient wear-resistance.
To relieve the molybdenum carbide-based wearresistant coating of residual stresses the cutting tool is subjected to a stepped vacuum annealing. It should be noted that the vacuum annealing results in partial loss of hardness of both the wear-resistant coating and the cutting tool itself, which affects adversely its service durability.